UPPER FLOW REGIME BEDFORMS AND THE 'MEGAMONSOON' HYPOTHESIS OF THE TRIASSIC DOCKUM GROUP, WEST TEXAS

The Triassic Dockum Group of the West Texas High Plains records the dryland fluvial and lacustrine systems of Pangaea during the Triassic. These strata contain a rich record of vertebrate life and, therefore, have been the focus of many paleontological studies. Despite this, the fundamental sedimentology of the Dockum Group lacks an updated depositional framework with consideration to modern understandings of the systems that dominated it. This study focuses on outcrops of the Tecovas, Trujillo, and Cooper Canyon formations of the Dockum Group situated along the eastern escarpment of the High Plains north of the Matador Arch.

Dockum Group formations host six lithofacies assemblages: upper flow regime channel complexes, upper flow regime sheets, perennial channel complexes, floodplains, paleosols, and lacustrine deposits. Upper flow regime channels are distinguished by upper plane beds, antidune cosets, chutes and pools, and cyclic steps, whereas upper flow regime sheets are dominated by upper plane beds. These lithofacies assemblages were previously undocumented in the Dockum Group. Fluvial architectural analysis was conducted on each lithofacies assemblage in order to highlight the internal geometries and define the depositional processes.

The depositional patterns of each lithofacies assemblage reveal a “mega monsoonal” climate that perpetuated throughout the Triassic. The climate was generally dry but subject to intermittent high rainfall events. Upper flow regime assemblages were deposited in megafloods under intense monsoonal rains as well as local storm fronts interrupting dry seasons. Both floodplains and small lacustrine systems were inundated and filled during the rainy seasons, then subsequently desiccated during the dry months. The Dockum Group also hosts larger and deeper lakes that persisted for some significant time between the floods.

This study identifies and interprets previously unobserved structures and bedforms in the Dockum Group and therein derives implications to the violent weather patterns during the Triassic. The pressure differential associated with a landmass as impressive in size as the supercontinent of Pangea created storms of such immense size and strength that these “mega monsoons” pushed through the prevailing westerlies and onto the continent. The rarely preserved upper flow regime structures in the Dockum Group suggest monsoonal rains such as the modern world has never seen.